Typically, commercial publication type gravure dryers have consisted of a recirculating air system in which a fan blows controlled temperature air through nozzle orifices against the wet ink on the paper web to produce a drying action. The spent air then returns to the same fan for repeated cycling. Concurrently an exhaust air system operates which taps off part of the return air going to the recirculation fan, using a second (exhaust) fan to eject it from the dryer system either to atmosphere or to some type of emission control system such as solvent vapor recovery. Makeup air to replace that exhausted leaks in from the room through the web entering and leaving slots.
It is known that a large percentage of the solvent vapors are liberated at or just ahead of the web entrance to the dryer so a separate duct is connected from the web entrance slot directly to the plenum from which exhaust and recirculated air are channeled to their respective fans. However, the fact that in all of the various models it has been customary to direct air to the recirculation fan inlet from this common plenum gives rise to a solvent vapor concentration problem. Although it is desirable to imping air on the web for drying purposes with as low a solvent vapor concentration as possible, there dryers actually use solvent air of excessively high concentration since it contains a great percentage of the air taken directly from the web inlet slot where the greatest solvent evaporation takes place. To compound the problem, it is found that the internal circuitry is so arranged that there is substantial shortcircuiting of low concentration air coming in through the web outlet slot directly to the exhaust duct.
Some prior art dryers present a safety problem. With both recirculated and exhaust air drawn from a common chamber, it is possible for the negative pressure from the recirculation fan to reach into the exhaust duct and produce a "safe" indication at the exhaust static pressure safety switch even though the exhaust fan, being monitored by this switch, is not running.
Some prior art patents which have systems of louvers for proportioning the recirculated air and the exhausted air have suggested that all spent air go to exhaust with drying being done only by new air brought into the system. This, however, is very wasteful of fuel because none of the air which has been heated before it impinges upon the web is recirculated, causing a dryer operated in that manner to have excessively high energy requirements for heating the air before it impinges the web.
The most pertinent patents of which applicant is aware are U.S. Pats. No. 2,225,505, issued Dec. 17, 1940 to Bernard Offen, and U.S. Pat. No. 2,226,319, issued Dec. 24, 1940 to Bernard Offen.